Gas deposition is used in a variety of applications for depositing a material onto a target surface of a work piece such as a semiconductor wafer or magnetic storage media. The materials are deposited for a variety of reasons such as to form thin-film surfaces, silicon barrier layers, and protective coatings for semiconductor feature characterization and analysis. Regarding the latter, cross-sectional slices are cut out from the surface at an area of interest on the wafer to expose an interior, cross-sectional face for imaging. An imaging device such as a scanning electron microscope (“SEM”) then images the cross-sectional interface, in order to analyze, characterize, or measure feature dimensions within it. Typically, the cross-sectional slice is removed from an area on the wafer surface that contains at least a portion of a feature of interest such as a polygate or photo-resist line or contact. The protective layer is needed on the surface when the slice is being cut in order to shield the area around the cut and preserve the characteristics of the feature's interior portion that are to be imaged (e.g., for measurement).
A gas deposition material is generally deposited onto a work piece surface in the following manner. A charged particle (or other) beam is applied to the target surface in the presence of a deposition gas. Upon striking the surface, the charged particle beam causes the surface to emit secondary electrons, which interact with the deposition gas causing it to form deposition particulate. As this goes on, the particulate accumulates on the underlying target surface thereby forming the deposition layer. Many combinations of gasses, work piece surfaces and beam types can be used to achieve a variety of deposition schemes. For example, commonly used deposition gasses include those that contain tungsten, platinum, gold, and carbon for depositing the same onto the target surface. The particular material to be deposited will usually depend on the application, underlying target surface, and how the material reacts with the same. Similarly, a variety of beam types can be used to generate secondary electrons. These include ion, electron, and laser beams. Depending on the target surface material and its secondary electron emission coefficient in connection with the particular beam type and parameters, any of these beam types may be preferred in a given application. With feature characterization and measurement applications involving silicon based wafers, a tungsten gas with a gallium ion beam are commonly used to apply tungsten as the protective layer over the target surface.
FIG. 1A is a top view of a semiconductor wafer 100 that includes a line feature 105. In order to measure aspects of the line feature in the Z-axis, a cross-sectional slice is typically cut away from the feature 105 and wafer surface (along the X-axis) to obtain a cross-section for measuring X-Z characteristics of the feature. Typically, a focused ion beam (“FIB”) is used to cut away the cross-sectional sliced. A SEM is then normally used to measure the X-Z characteristics of the feature's cross-sectional face. In fact, dual beam systems with both a FIB and a SEM are commonly used in the semiconductor industry to more efficiently perform these functions with a single device. However, as it is well known in the art, while FIB beams work well for efficiently cutting away cross-sectional slices, they are typically not “clean” enough to cut the slice without damaging at least part of the surrounding area including important aspects of the feature to be measured. Thus, a protective deposition layer, as discussed above, is deposited on the wafer 100 and feature 105 surfaces before cutting away the cross-section.
With reference to FIG. 1B, it is common to use the FIB, itself, for depositing the protective layer. Since it is used for cutting away the cross-sectional slice, it is already in place to irradiate the target surface for depositing the deposition layer. To do this, the FIB beam 103 is scanned over the target surface including the relevant wafer and feature surfaces, in the presence of a suitable deposition gas 120 diffused proximal to the target surface, to deposit the protective material onto both the feature and wafer surfaces within the target surface. Upon striking the target surface, the FIB beam 103 causes secondary electrons, e, to be emitted in all directions from the point on the target surface that is struck by the beam. The electrons interact with the gas causing a particulate to form and be deposited onto the surface below. As shown in FIG. 1C, when the process is completed, a protective layer 125 formed from the deposited particulate results over the target surface.
Unfortunately, when the FIB is initially scanned onto the target surface, it sputters material away from the surface for a period of time until a sufficient amount of deposition material accumulates to shield the exposed feature surface from the FIB. Even though this time may be small;, it can be large enough to allow a significant amount of material to be removed, which causes the accuracy of the cross-sectional analysis to be compromised. For example, as shown in FIG. 1C, the line feature 105 becomes fairly rounded at its upper, outside edges, which incur the greatest un-shielded exposure to the FIB. This can be highly problematic, especially with decreasingly smaller dimensions and tolerances used in today's chip manufacturing processes. For example, with the depicted “rounded” line feature, its width might incorrectly measure as being small enough for the line feature to wrongfully be characterized as being out of tolerance.
As mentioned above, electron and laser beams can be used to generate the electrons necessary for material deposition, but they may also damage the underlying surface—especially when they are at sufficient energy and/or current density levels for achieving favorable throughput. However, it is normally not practical to use them because they will typically be too slow if “weak” enough not to harm the underlying surface. Moreover in many environments such as in dual beam systems, for example, they are not properly aligned for scanning the target surface at suitable angles for gas depositioning. Plasma vapor deposition (“PVD”) sputter methods could be acceptable in some applications, but they normally cannot be utilized for FAB production control applications because they cannot be used to locally apply a deposition layer onto a targeted part of the wafer surface.
Accordingly, what is need is an improved method and system for depositing materials onto a target surface.